Pressure differential sensor in which pressure is transferred to a measuring element by means of a pressure-transfer liquid, especially such pressure difference transducers having an excess-load, or overload, membrane, or diaphragm will be considered. Such a pressure difference transducer is disclosed, for example, in the Offenlegungsschrift (laid open German application) DE 196 08 321 A1. Pressure difference transducers include a hydraulic body, in which an overload chamber is formed. The overload chamber contains an overload membrane. The overload membrane divides the overload chamber into a high-pressure half-chamber and a low-pressure half-chamber. The high-pressure half-chamber is connected into, i.e. communicates with, a first hydraulic path; the first hydraulic path extends between, on the one hand, a first diaphragm seal involving a separating membrane, or diaphragm, over a membrane bed, and, on the other hand, a high-pressure side of a pressure measuring cell. The low-pressure half-chamber communicates with a second hydraulic path, which extends between, on the one hand, a second diaphragm seal involving a separating membrane over a membrane bed, and, on the other hand, a low-pressure side of the measuring element. Other terms used in the art for such diaphragm seals include “chemical seal”, “pressure intermediary”, and “pressure mediator”.
In the case of large overloads, the pressure-transfer liquid is forced completely out of the diaphragm seal, and the separating membrane lies on the membrane bed. The pressure-transfer liquid displaced in the relevant hydraulic path in the case of overload pressure leads to a deflection of the overload membrane. This absorbs the additional volume and reduces the overload pressure.
The spring stiffness of the overload membrane is a function of operating temperature range, the possible system pressure, the volume of the hydraulic, pressure-transfer liquid, and the overload factor of the measuring element. The overload factor defines how much the overload pressure can exceed the measuring range before the separating membrane comes to rest, and no further pressure increase occurs at the measuring chip. The greater the overload factor of the measuring element, the stiffer the overload membrane can be made. The stiffer the overload membrane, the faster the measuring cell reacts to pressure fluctuations, that is, the cell then reacts faster to fluctuations in pressure. This is especially important in the case of sensors having diaphragm-seal add-ons connected with the sensor via long capillary lines. A long capillary line has a large hydraulic resistance and, with the overload membrane, forms an RC-element. In the case of a soft overload membrane having a large hydraulic capacitance C, this can lead to large time constants, and respectively long response times.
The spring stiffness of the overload membrane is generally symmetric in the HP and LP directions. The overload factor of the measuring element is clearly different between HP and LP loading. The design of the symmetrical overload membrane depends on the lower burst value of the measuring element, a factor which unnecessarily increases the time constant of the high-pressure side of the pressure difference transducer.